Proximity sensing systems aimed at object detection in general are well known per se and are found in wide use in commercial, police and military applications. Generally, such systems fall into two broad categories of systems: “active” systems based on signal transmission towards an object and detection of signals returned (reflected) from the object, and “passive” systems that do not utilize energy transmission towards a target to be detected (e.g., infrared or heat-seeking systems).
U.S. Pat. No. 4,185,192 discloses a passive optical system utilizing two detectors oriented such that their optical axes and cone shaped fields of view intersect, thereby creating an overlapping region between these fields of view. By this, only those signals from the detectors, which are received simultaneously, being thereby indicative of that the detected light comes from the overlapping region, will cause the generation of a switching signal.
Another example of such a passive optical system for determining the presence of an object by utilizing the creation of a scene at the intersection of two optical paths associated with two detectors is disclosed in U.S. Pat. No. 4,317,992.
U.S. Pat. No. 4,396,945 discloses the technique that can be utilized in either an active or passive system for determining the position and orientation of an object in space. This technique is based on the determination of the intersection of a line with a plane or with another line.
Active systems are also disclosed in U.S. Pat. Nos. 4,590,410 and 4,724,480. According to the technique of U.S.'410, which is aimed at detecting small objects, multiple light emitters and multiple light detectors are utilized. According to the technique of U.S.'480, a system is composed of at least one projector generating a non-planar light and at least one camera, which are oriented such that the optical axes of the camera(s) and projector(s) intersect. By this, the projector associated with a first object and the region of intersection associated with a second object can be aligned.